In the field of 3GPP that regulates technical standards of the third generation mobile communication system, since the end of 2004, has started researches for Long Term Evolution/System Architecture Evolution (LTE/SAE) techniques to optimize and enhance functions of 3GPP techniques in correspondence to a plurality of forums and new techniques relevant to the 4th generation mobile communication.
The SAE based on the 3GPP SA WG2 relates to a network technique for determining a network structure and supporting mobility of a heterogeneous radio network system with cooperating with an LTE operation of the 3GPP TSG RAN. The SAE, one of the most important standardization issues of the 3GPP, is implemented to develop a 3GPP system into a system that supports various wireless access techniques based on IP. More concretely, the SAE has been implemented for an optimized packet-based system capable of minimizing transmission delay with an enhanced data transmission capability.
A conceptual reference model of the SAE, defined by 3GPP SA WG2 includes a non-roaming case, and a roaming case having various scenarios. Details of the conceptual reference model can be referred from TS 23.401 and TS 23.402 which are 3GPP standard documents. This may be schematically reconfigured in FIG. 1.
FIG. 1 is a structural view of an evolved mobile communication network.
One of the most representative characteristics of the network of FIG. 1 is that a structure is based on a two-layer model (2 Tier Model), an evolved NodeB (so-called eNodeB) of an Evolved UTRAN and a Gateway of a Core Network. The eNodeB has similar functions to them of both a RNC and a NodeB of the conventional UMTS system. And, the Gateway has a similar function to it of the conventional SGSN/GGSN.
Another important characteristic of the network is that a Control Plane and a User Plane between an Access Network and a Core Network are interchanged to each other through different interfaces. In the conventional UMTS system, one interface (lu) exists between an RNC and an SGSN. However, since a Mobility Management Entity (MME) which processes a control signal is separated from a Gateway (GW), two interfaces (i.e, S1-MME and S1-U) were respectively used.
FIG. 2 shows an (e)NodeB and a Home (e)NodeB.
In the 3rd or 4th generation mobile communication system, efforts to increase a cell capacity have been ongoing in order to support high-capacity service such as multimedia contents and streaming, and a bi-directional service.
As various techniques for transmitting a large amount of data in addition to multimedia relating techniques are required, many methods for increasing wireless capacity have been researched. One of the methods include a method for allocating frequency resources as much as possible. However, there have been limitations in allocating limited frequency resources to a plurality of users as much as possible.
In order to increase a cell capacity, there are efforts to use a high frequency bandwidth, and to reduce a cell radius. When cells having a small radius, such as pico cells are used, a frequency bandwidth of the cell can increase highly than that in the conventional cellular system thus to transmit more information. However, in this case, more base stations have to be installed in the same area, which results in high costs.
In order to increase a cell capacity by using a small cell, a Home (e)NodeB 30 has been proposed.
The Home (e)Node 30 has been researched based on RAN WG3 of the 3GPP (e)NodeB, and is being intensively applied to SA WG.
Referring to FIG. 2, an (e)NodeB (or NodeB) 20 may correspond to a macro-base station, whereas a Home (e)NodeB (or Home NodeB) 30 may correspond to a femto-base station. In the specification, the terms will be explained based on the 3GPP. And, the (e)NodeB 20 will be used so as to indicate ‘NodeB’ or ‘eNodeB’, and the Home (e)NodeB 30 will be used so as to indicate ‘Home NodeB’ or ‘Home eNodeB’.
A cell of the Home (e)NodeB 20 is implemented in an Open Access Mode, a Closed Access Mode, and a Hybrid Access Mode.
In the case of the Open Access mode, the cell of the Home (e)NodeB 20 provides service to all serviceable terminals without limitations.
In the case of the Closed Access mode, the cell of the Home (e)NodeB 20 permits access of only allowed terminals.
In UMTS/EPS of the 3GPP standard, it has been proposed that one or more Home (e)NodeBs operated in the Closed Access mode forms one Closed Subscriber Group (CSG). That is, one CSG may be composed of one or more Home (e)NodeBs, and the terminal also receives a one permission (e.g., one CSG membership) to access the cells of the Home (e)NodeBs. Here, the terminal may have one or more CSG membership to access one or more CSGs, and may have time information allowed according to each CSG. Information on accessible CSGs is called as an Allowed CSG List. This allowed CSG list is stored in the terminal, and a network entity such as MME, SGSN, MSC, HSS, and HLR.
Access control is performed according to the allowed CSG list. For instance, if accessible time to the CSG has expired or a permission to access the CSG has been deleted (or expired) while the terminal receives service, a handover to a suitable cell is executed. That is, a handover to a peripheral CSG, or to a NodeB (or eNodeB) corresponding to a macro-base station is executed.
In the related art, if accessible time to the CSG has expired or a permission to access the CSG has been deleted, the terminal may execute a handover to a suitable cell. However, concrete methods thereof have not been disclosed.
In order to perform a handover from a first CSG to which the terminal is being accessed to a second CSG, a status of wireless resources (i.e. whether the terminal is accessible to the second CSG) has to be considered. However, this has not been disclosed in the related art.
For the consideration, signal transmission between relevant entities is required as much as possible. However, the related art has not disclosed appropriate solutions relating thereto, which causes waste of wireless resources.
Furthermore, even if the terminal has not found a suitable CSG, a handover to a macro-cell (NodeB or eNodeB) has to be executed so as to ensure service continuity. This may depart from the original purpose of the Home (e)Node B for reducing the amount of resources usage by the macro base station.